This invention relates to an improved process for making solid elastomeric copolymers of vinylidene fluoride and at least one other fluorine-containing monomer.
Elastomeric interpolymers of vinylidene fluoride and other copolymerizable fluorine-containing monomers, such as hexafluoropropene and/or tetrafluoroethylene, optionally containing small amounts of other fluorine-containing monomers, are well known in the art and have an established place in commerce because of their excellent chemical and thermal stability.
These elastomers are generally prepared by the aqueous emulsion polymerization of fluoroolefin monomers in the presence of water-soluble free radical initiators (e.g., ammonium persulfate) with surfactants and chain transfer agents by either batch or continuous polymerization procedures. However, residual amounts of surfactants remaining in the fluoroelastomers have an adverse effect on the physical properties of the resulting fluoroelastomers due to the presence of acid end groups on the surfactant in the polymer. It is difficult to remove the surfactant from the fluoroelastomers, as contrasted to removing surfactants from fluoroplastics, because the fluoroelastomers cannot be heated to temperatures sufficiently high to remove the surfactant by volatilization without, at the same time, decomposing the fluoroelastomers. The present invention is directed to a process for removing substantially all of a particular group of surfactants from the fluoroelastomer while retaining the beneficial properties of the fluoroelastomer, such as a high compression set resistance, good stress-strain properties and low die swell. The present process results in a fluoroelastomer having few acid end groups. The presence of acid end groups detrimentally effects the processing characteristics of the fluoroelastomer since these groups increase the viscosity of the polymer and interfere with curing systems, especially those based on quaternary phosphonium salts.